Uniaxial Behavior Research Articles

Constitutive modeling of strain-dependent bond breaking and healing kinetics of chemical polyampholyte (PA) gel.

A finite strain nonlinear viscoelastic constitutive model is used to study the uniaxial tension behaviour of chemical polyampholyte (PA) gel. This PA gel is cross-linked by chemical and physical bonds. Our constitutive model attempts to capture the time and strain dependent breaking and healing kinetics of physical bonds. We compare model prediction by uniaxial tension, cyclic and relaxation tests. Material parameters in our model are obtained by least squares optimization. These parameters gave fits that are in good agreement with the experiments.

Time-dependent uniaxial behavior of rolled magnesium alloy AZ31B at 393 K and room temperature

To address the time-dependent properties of rolled AZ31B alloy, we conducted typical tests of the rate jump, creep, and stress relaxation at room temperature and 393 K. In the rate jump tests, the tensile curve exhibited a strong dependence on the strain rate, whereas compression was totally insensitive to the stress rate at both temperatures. For the creep and stress relaxation test, we observed creep strain and decay stress in the compression, which was weaker than the tensile curve. The plastic viscosity increased at 393 K because the dislocation motion was thermally activated. We then applied thermal activation theory for the repeated stress relaxation tests. The activation volume implies that cross-slip and dislocation nucleation are the operating mechanisms for creep and stress relaxation.

A finite element model for the deformation of floating ice covers

In many practical situations, ice is very close to its melting point and can be represented mechanically as a viscoelastic material. Based on small-scale experiments conducted on S2 ice over a number of years, Sinha (1978a, 1978b, 1979) developed a representation of uniaxial ice behaviour that incorporates elastic deformation, recoverable primary creep (delayed elastic) deformation and permanent secondary creep deformation. In subsequent work, Zhan (1993) and Zhan et al. (1994, 1996) extended the representation to multiaxial situations.In this paper, a continuum finite element solution is developed based on Sinha's approach for two-dimensional plane stress situations that are appropriate for solving practical problems involving S2 freshwater and sea ice deformation in the plane of the ice cover. The ice behaviour is considered to be isotropic with respect to elastic and creep deformations, and creep deformations in the vertical direction are assumed to be negligible. A particular feature of the present model is the consideration of pre-existing macrocracks or gaps at the shoreline in landfast ice. The present model does not account for the evolution of cracks.Following a calibration of primary creep parameters to Sinha's model and a small-scale experiment on sea ice, examples are presented to illustrate landfast sea ice deformation as a result of wind forces, shoreline displacements due to the tidal jacking mechanism, and restraint at the shore.

Micromechanical modeling of the multi-axial deformation behavior in double network hydrogels

The Mullins-type damage behaviors in double network hydrogels have attracted a broad research interest in recent years. However, most of current works focus on characterizing and modeling the uniaxial deformation behaviors of these materials. In this work, we combine experimental and theoretical approaches to investigate the anisotropic damage behaviors of double network hydrogels revealed in multi-axial deformation conditions. We demonstrate that an isotropic damage model based on the eight-chain model and the network alteration theory fails to capture the stress response in multi-axial loading tests. An anisotropic damage theory based on the microsphere model has also been developed, while both the affine and non-affine approaches are adopted to obtain the micro-macro mapping. The results show that the affine microsphere model cannot describe the experimental results in pure shear and unequal biaxial tests. Remarkably, the non-affine microsphere model with three parameters captures all the important features of the experimental observations. This is because the non-affine model accurately predicts the directional damage of the primary cross-linked network. The non-affine microsphere model is also able to describe the damage cross-effect in double network hydrogels. The developed theoretical framework can promote the fundamental understanding of the anisotropic damage behaviors in various types of tough gels.

Evaluation of tensile properties of various regions of creep exposed dissimilar weld joint using miniature specimen testing

Strength variations in various regions of a dissimilar weld joint made between 316LN stainless steel (SS) and P91 steel after creep exposure at 550 °C and stress of 200 MPa was estimated using automated ball indentation (ABI) test. The ABI results were used to derive equivalent uniaxial tensile flow behaviour using established correlation factors. A comparison of the results obtained in the creep tested and post weld heat treated (PWHT) samples showed loss of strength in the P91 steel side. It was observed that, creep exposure under this testing condition strengthened the 316LN SS side of the weld joint. ABI derived results showed that, the inter critical-heat affected zone (IC-HAZ) of P91 steel is the weakest region across the weld after creep exposure. The narrow IC-HAZ created a strain constraint which caused the specimen to fail in the P91 steel base metal region.

Temperature- and time-dependent behavior of Z2CN18.10 stainless steel under uniaxial loading

A series of uniaxial monotonic tensile tests were first conducted on the piping specimens of Z2CN18.10 stainless steel within the temperature range from 20°C to 350°C at different strain rates. The results show an interesting characteristic of the material: it exhibited obvious rate-dependence at room temperature, whereas at 350°C no such significant dependence was observed. Then some tests were performed within the same temperature range to explore the effect of this property on the uniaxial ratcheting behavior of Z2CN18.10 steel. The results reveal that with increasing temperature the ratcheting behavior of the material tended toward shaking-down and became less sensitive to loading rates and peak dwell time. Such temperature-dependent viscoplasticity of the material can presumably be attributed to the dynamic strain aging (DSA) effect, which results in more loss of viscosity of the material as temperature increases.

Mesoscale modeling of recycled aggregate concrete under uniaxial compression and tension using discrete element method

This paper presents a computational study to underline how the uniaxial behaviors of recycled aggregate concrete (RAC) are affected by various factors. Particularly, RAC is modeled using a discrete element method (DEM) wherein each phase of RAC (i.e., new and old aggregates, mortars, and in-between interfaces) is represented explicitly. By doing so, the mesoscopic cracking process of RAC can be accurately traced. The development, calibration and validation of the modeling method are presented. Then a parametric study is conducted, considering the following major factors: the attributes of new mortar and old mortar, the old mortar content, the severity of damages produced in manufacture of recycled aggregate (RA), and the bond property of interfacial transition zones (ITZs). It is shown that the relative strength of old mortar to new mortar plays a vitally important role in defining the failure mechanism of RAC. Specifically, if old mortar is stronger than new mortar, cracking initiates at their interfaces. By contrast, for relatively weak old mortar incipient cracks arise instead from inside of RA particles, localizing mostly at old aggregate–old mortar ITZs. Yet in some extreme cases with very weak old mortar, early damages are trapped in old mortar alone. Those different origins of crack formation affect the macro-level behavior of RAC considerably. Moreover, the relative amount of old mortar has a significant effect—a higher content of such constituent reduces both strength and modulus of RAC, but concurrently delays the onset of mesoscale cracking. Results also show that a weak ITZ between RA and new mortar confers a pronounced reduction in RAC’s mechanical properties; conversely, the properties’ enhancement from improving those ITZs is capped though. This research illustrates how the developed DEM-based approach can be conducive to a better understanding of RAC’s mechanical response to uniaxial loadings.

A new crystal plasticity modeling of uniaxial ratcheting behavior for face-centered cubic 6061 aluminum alloy

The uniaxial ratcheting behavior of face-centered cubic 6061 aluminum alloy is investigated theoretically based on the crystal plasticity theory. In this model, a simplified flow rule is adopted for convenient engineering application, and the classical Kocks-Mecking-Estrin isotropic hardening rule related to dislocation density is adopted to describe the cyclic hardening characteristic of the material. The classical Armstrong-Frederic kinematic hardening rule is modified associated with cumulative slip to predict the ratcheting behavior more accurately. The single crystal version of this model is expanded to metal polycrystalline using a simplified explicit scale transition rule. The capability of the proposed model to capture the uniaxial ratcheting response of metal polycrystalline is verified by comparing the predictions with the corresponding experimental results of 6061 aluminum alloy. The evolution of uniaxial ratcheting of 6061 aluminum alloy can be reasonably predicted by the proposed model, and the capability to simulate uniaxial ratcheting behavior in inter-granular scale is qualitatively discussed as well.

The effect of initiator, polyfunctional monomer and polybutene-1 resin on the long chain branching of random polypropylene copolymer via reactive extruder

ABSTRACT Polypropylene random copolymer (PPR) and polybutene-1(PB-1) are blended and long-chain branched (LCB) in the reactive extruder. The optimum amount of PB-1 resin, peroxide (DCP) and trimethylolpropane triacrylate (TMPTMA) for long-chain branching of PPR and probe topology formed by the branches were investigated. Fourier transformed infrared spectroscopy (FTIR) used to measure the branching degree and efficiency as well as obtaining TMPTMA grafts on PPR. Oscillatory shear rheological properties including complex viscosity, storage modulus, loss tangent, and uniaxial elongational rheometry were studied to differentiate the LCB-PPR from unmodified PPR with almost similar molecular weight samples. The LCB structure modified samples with TMPTMA and DCP, prominent higher zero-shear viscosity, longer relaxation time and pronounced strain-hardening behavior. It was found that different amount of DCP and TMPTMA can affect strain-hardening behavior and topology of branches and subsequently the foaming ability. PB-1 resin found to have a positive effect because of more sensibility to β-scission degradation and mobilizing effects. The optimization of PB-1 for the highest zero-shear viscosity, shear, and uniaxial rheological behavior is achieved at 10 wt% PB-1, while 0.08% of DCP and 3% of TMPTMA are optimum amounts of additives needed for long-chain branching process. The optimum blend could be a great alternative for foaming applications.

Uniaxial stress relaxation behavior of marble after cyclic disturbance loads

It is known that time-dependent rheological deformation of rocks is influenced by cyclic disturbance loads. However, the stress relaxation behavior after cyclic disturbance is still poorly understood. To study such effects, a dynamic disturbance electrohydraulic servo triaxial test system was developed. A series of disturbance relaxation tests with different disturbance numbers and amplitudes were carried out on marble at several strain levels. The influence of cyclic disturbance parameters on the relaxation magnitude, relaxation rate, and peak stress was analyzed. Relaxation magnitude and relaxation rate after cyclic disturbance loads are larger than those of conventional relaxation. In addition, sensitivity analysis reveals that the relaxation magnitude is more sensitive to the disturbance amplitude. Damage variables defined by the plastic strain method can well reflect the evolution law of relaxation damage under different disturbance conditions. It is believed that the damage model constructed by Harris function can better describe the relaxation damage evolution law at various strain levels.

Biocompatible, Flexible Strain Sensor Fabricated with Polydopamine-Coated Nanocomposites of Nitrile Rubber and Carbon Black.

A flexible, biocompatible, nitrile butadiene rubber (NBR)-based strain sensor with high stretchability, good sensitivity, and excellent repeatability is presented for the first time. Carbon black (CB) particles were embedded into an NBR matrix via a dissolving-coating technique, and the obtained NBR/CB composite was coated with polydopamine (PDA) to preserve the CB layer. The mechanical properties of the NBR films were found to be significantly improved with the addition of CB and PDA, and the produced composite films were noncytotoxic and highly biocompatible. Strain-sensing tests showed that the uncoated CB/NBR films possess a high sensing range (strain of ∼550%) and good sensitivity (gauge factor of 52.2), whereas the PDA/NBR/CB films show a somewhat reduced sensing range (strain of ∼180%) but significantly improved sensitivity (gauge factor of 346). The hysteresis curves obtained from cyclic strain-sensing tests demonstrate the prominent robustness of the sensor material. Three novel equations were developed to accurately describe the uniaxial and cyclic strain-sensing behavior observed for the investigated strain sensors. Gloves and knee/elbow covers were produced from the films, revealing that the signals generated by different finger, elbow, and knee movements are easily distinguishable, thus confirming that the PDA/NBR/CB composite films can be used in a wide range of wearable strain sensor applications.

Fabrication and uniaxial compression mechanical behavior of composite corrugated-core sandwich cylinder with thin-wall metal liner

The strong, stiff and weight efficient composite cylindrical shells with metal liner can withstand axial load, transverse load and higher internal pressure. In this paper, a composite corrugated sandwich cylinder with thin-wall metal liner was fabricated by integrated-solidification. Then, the damage and failure modes of the sandwich cylindrical shells were tested by quasi-static uniaxial compression tests. Finally, an equivalent monocoque shell theory was applied to predict the multi-mode failure by simplifying the sandwich cylinder to a quasi-isotropic cylinder. And the theoretical method could predict the failure modes of the sandwich cylinder within acceptable errors.

Molecular dynamics study on axial mechanical properties of calcium silicate hydrate

The axial mechanical properties of calcium silicate hydrate during uniaxial tension or stress have been investigated by molecular dynamic simulations. Based on the classic calcium-silicate-hydrate model, the influence of calcium to silicate ratio and temperature are studied in detail. It demonstrated that calcium silicate hydrate with different Ca/Si ratio have similar stress-strain curves under uniaxial compression, but greatly difference on the uniaxial tension behaviors. The uniaxial tensile properties was affected by temperature that the strain rate in the low-temperature is higher than that in high temperature. The work in this paper reveals the mechanical properties of calcium silicate hydrate at the nanoscale and establishes the foundation explore the failure mechanism of cement or cement-based materials.

A nonlinear uniaxial stress-strain constitutive model for viscoelastic membrane materials

Membrane materials with the excellent thermal, optical, electrical and chemical properties have attracted significant attention in numerous research fields recently. However, while being used to construct the membrane structures, the mechanical behaviors of membrane materials are more foundational than the other properties in evaluating the structure safety. This paper thus proposes a nonlinear stress-strain constitutive model for revealing the viscoelastic behaviors of membrane materials under uniaxial tensile loading. To this end, the constitutive equations for expressing the uniaxial tensile stress-strain relationships of viscoelastic materials are established gradually from the kinematic equations of the generalized Maxwell model that includes several basic Maxwell models and one basic spring element. Meanwhile, the uniaxial tensile tests of two typical viscoelastic membrane materials were carried out in order to examine the proposed constitutive model. The constitutive model parameters of the stress-strain properties of both membrane materials are accurately identified using the least square method. By comparing the true stress-strain curves between experimental results and constitutive models, good agreements with the maximum differences of 4.67% and 3.41% are acquired for the two employed viscoelastic membrane materials, respectively. These observations are able to validate the accuracy and efficiency of this proposed constitutive model in predicting the uniaxial stress-strain behaviors of viscoelastic membrane materials, which are significant in the nonlinear structural analysis of membrane structures.

Low-Cycle Fatigue and Ratcheting Lifetime Estimation of a 7050-T6 Aluminum Alloy

Low-cycle fatigue properties and uniaxial ratcheting behavior of a 7050-T6 aluminum alloy were investigated by a non-symmetric cycle stress tests at the normal temperature. The low-cycle fatigue behavior of the alloy was evaluated. The effect of the stress rate, stress amplitudes, and mean stresses on its ratcheting performance was discussed. Considering the ratcheting effect and special working conditions that large mean stresses and small stress amplitudes, based on the same properties under loading, a new fatigue life prediction empirical equation was proposed to estimate the ratcheting life. The results prove that the ratcheting strain of the alloy depends on the stress amplitudes and mean stresses, whereas the stress rate almost has no effect on the ratcheting behavior on account of the rate-independent feature of the material. This equation can give a precise estimate of the ratcheting behavior of large mean stresses and their small amplitudes.

Optimization procedure for parameter determination of caboche kinematic hardening model

The ratcheting behavior and prediction of 316L stainless steel pipe was developed using numerical finite elementapproach. The Chaboche model parameters to be used in the numerical method were adopted from symmetric post monotonic teston the pipe. The significant property, the elastic limitof the material,was extracted using incremental cyclic uniaxial test. Comparison between two well-known optimization methods, Particle Swarm Optimization and Genetic Algorithm gave the former a slightly better correlation with experimental tests.The Chaboche model in ANSYS was utilized to predict the uniaxial ratcheting behavior of the pipe specimen. Theclose comparison of rate of ratcheting between finite element simulation and experimental test indicates that accurate elastic limit extraction from post monotonic test alone results in improved ratcheting prediction, with PSO method adopted to optimize ratcheting parameters.

Modeling the mechanical behavior of coarse-grained soil using additive manufactured particle analogs

Systematic investigation of the effects of individual particle properties, such as shape, size, surface roughness, and constituent materials stiffness, on the behavior of coarse-grained soils requires careful control over the other particle properties. Achieving this control is a pervasive challenge in investigations with naturally occurring soils. The rapid advance of modern additive manufacturing (AM) technology provides the ability to create analog particles with independent control over particle size and shape. This work evaluates the feasibility of the stereolithography (SLA) and polyjet technologies to generate analog particles that can model the mechanical behavior of coarse-grained soils. AM is used to generate equal-sized spheres and analog sand particles from 3D X-ray CT scans of natural rounded and angular sand particles. The uniaxial inter-particle compression, oedometer compression, and shear wave transmission behaviors of the AM particles are investigated and compared to those of glass and steel spheres and natural rounded and angular sand particles. The results indicate that AM can successfully reproduce the shape of natural coarse sand particles. The deformation of micro-asperities was found to influence the contact response of the polyjet AM particles, thus affecting their inter-particle uniaxial compression and oedometer compression response. The contact response of the SLA AM particles was closer to that of glass spheres. Both AM particle types exhibit a dependency of shear wave velocity and shear modulus on mean effective stress that is consistent with that of natural sands.

Stress-controlled LCF experiments and ratcheting behaviour simulation of a nickel-based single crystal superalloy with [001] orientation

Uniaxial ratcheting behaviour and low cycle fatigue (LCF) failure mechanism of nickel-based single crystal superalloy DD6 with [001] orientation are investigated through the stress-controlled LCF tests with stress ratio of −1. Then the deformation behaviour during the whole-lifetime from the beginning of the experiment to the fracture of the specimen, as well as the fractographic/metallographic morphology, are compared with the strain-controlled LCF experimental results. Through the scanning electron microscope (SEM) observations, it is shown that the failure characteristics under stress-controlled LCF loading are similar with those under strain-controlled loading. Nevertheless, unlike strain-controlled LCF loading, even under fully reversed cycle loading for stress-controlled LCF, DD6 shows significant ratcheting behaviour due to the tension-compression asymmetry. In addition, the LCF lifetimes under stress control are significantly shorter than the LCF lifetimes under strain control, and the culprit might be the detrimental effect of ratcheting strain on LCF lifetime. Based on these phenomena, an improved crystal plasticity constitutive model on the basis of slip-based Walker constitutive model is developed through modifying the kinematic hardening rule in order to overcome the inaccurate prediction of decelerating stage and stable stage of ratcheting behaviour. Furthermore, combining the continuum damage mechanics, a damage-coupled crystal plasticity constitutive model is proposed to reflect the damage behaviour of DD6 and the accelerating stage of ratcheting behaviour. The simulation results for the stress-controlled LCF deformation behaviour including the whole-lifetime ratcheting behaviour show good agreement with the experimental data.

Effect of cyclic frequency on uniaxial ratcheting behavior of a textured AZ31B magnesium alloy under stress control

The ratcheting behavior of magnesium alloy AZ31B was investigated under controlled cyclic stress at frequencies of 0.5 Hz, 1.0 Hz and 2.0 Hz. The result showed that cyclic loading accelerates strain accumulation as compared with static creep at room temperature, and the effect of cyclic frequency on strain rate dεr/dt, represented by strain in a unit time, is of multiplicity due to the diversification of deformation mechanisms. However, strain rate dεr/dN, represented by strain in one cycle, decreases evidently with increasing frequency at steady strain accumulative stage. High frequency promotes twinning-detwinning, and is favorable for prolonging the fatigue life in terms of cyclic number to failure. The fatigue life increases linearly with decrease of plastic strain energy density under all frequency conditions. This leads to conclude that frequency affects the fatigue life via changing plastic strain energy stored in the deformed microstructure.

Crystallisation behaviour of composites of HDPE and MWCNTs: The effect of nanotube dispersion, orientation and polymer deformation

Multi-walled carbon nanotubes (MWCNTs) were readily dispersed and distributed in the high density polyethylene (HDPE) matrix and a very low (<0.3 wt%, 0.17 vol%) rheological percolation threshold obtained. The polymer is seen to coat the MWCNTs, which act as a nucleating agent and induce an oriented nanohybrid shish-kebab (NHSK) crystalline structure, in part an artefact of the injection moulding process. The elastic modulus and yield stress of HDPE increased on addition of MWCNTs, but the polymer strain at break increased sharply below percolation before decreasing again above percolation. Addition of MWCNTs, above the percolation threshold, hindered orientation of the polymer chains such that failure occurred before necking during tensile testing, the inclusion of MWCNTs altered the uniaxial deformation behaviour of HDPE. SAXS/WAXS studies on samples post uniaxial drawing revealed that the HDPE and composites of HDPE and MWCNTs developed a highly oriented sheared herringbone lamellar structure. The drawing process also caused surface cavitation and voiding in the composites.